1. Field of the Invention
The present invention relates to an image processing method and an image processing apparatus capable of enhancing a quality of an image output by an image output device represented by a color printer.
2. Description of the Related Art
In recent years, an image forming apparatus (image output apparatus) of an electrophotographic system has been represented by a laser printer or the like which changes intensity of a laser beam based on digitized image data to reproduce gradation, and put to practical use. Further, in the image forming apparatus of the electrophotographic system, resolution of an output image has been enhanced, and a resolution of 600 dots per inch (dpi), 1200 (dpi) can be obtained. In the image forming apparatus of the electrophoto-graphic system, an apparatus capable of outputting a color image by arrangement of a plurality of image forming units corresponding to color components color-decomposed in accordance with subtractive primaries or repetition of an image forming process has been put to practical use.
In general, when displaying (outputting) the color image and handling image data of signal processing, an RGB system is utilized in a monitor or the like which is a part of a computer, and a CMY system or a CMYBk system to which a black signal is independently added to CMY is utilized in a printer or the like.
In an image output apparatus represented by the printer or the like, to reproduce a half-tone image, pseudo gradation (half-toning) processes such as a dither process by the use of a threshold matrix and a concentration pattern process are utilized. In this type of pseudo gradation process, binary dot on/off outputs are controlled in a certain micro area with a pixel unit or a unit obtained by further dividing a pixel to thereby represent gradation with a unit area of a portion in which dots are on (present) (area gradation).
To output the color image, in the area gradation, four colors of cyan (C), magenta (M), yellow (Y), and black (Bk) or three colors of C, M, Y are repeated, and finally superimposed on an output medium. A subtle color gradation image can be reproduced by this method. It is to be noted that, in many cases, Bk obtained by superimposing three colors of C, M, Y has a color characteristic different from that of ideal Bk. Therefore, in the color image output apparatus of the electrophotographic system, in many cases, Bk is independently added to three colors C, M, Y.
Documents, graphics, photographs and the like prepared or edited by a host computer, a personal computer and the like are converted into page description languages (PDL) such as postscript and PCL language. The converted documents, graphics, photographs and the like are supplied to the image output apparatus via relay means such as LAN and centronics.
In the image output apparatus, the language input in a controller section in an image processing apparatus disposed inside the image output apparatus or independently is interpreted, and developed into raster data correlated with a position of an actual output image (development process (RIP (process)) from the postscript or PCL language into the raster data is performed).
In general, during the RIP (process), as image data output from the controller section, image data having a gradation reproduction capability adapted to a printing capability of the output apparatus is output at 1 bit to 7 bits per pixel (color). In the RIP of 1 bit or low bit, the bit is compressed in a half-toning process section in the controller section (a compression process of the bit is performed). In the 8 bits RIP (process), the bit is not compressed in the half-toning process section of the controller section, and the gradation is reproduced by modulation (pulse width modulation or power modulation) of the intensity of the laser beam on the side of an engine (output apparatus). From a viewpoint of image quality, in principle, when the number of bits of the RIP increases, there is a high possibility that a high-quality image can be reproduced.
Various systems have heretofore been proposed as an algorithm of the half-toning process which largely influences the image quality. The algorithm of the half-toning process is largely classified into AM modulation (periodic modulation) and FM modulation (frequency modulation). In the AM modulation, a size of a half-tone dot having a basic size is modulated with a given period, angle, and shape for each gradation, and the gradation is reproduced. The FM modulation is a system in which an average distance between the half-tone dots each having a magnitude (size) fixed for each gradation is changed to reproduce the gradation. A broadly known error diffusion process is also regarded as one type of FM modulation in some case.
When these methods are applied to the image output apparatus of the electrophotographic system, the following problems occur.
In the image forming apparatus of the electrophotographic system, it is difficult to form a single pixel (e.g., 600 dpi) in a stable state equal to the resolution of a printer (one dot of 600 dpi is reproduced with the printer of 2400 dpi). Therefore, it is known that the image is degraded with use of the dither matrix of the FM modulation type. Therefore, in many image forming apparatuses of the electrophotographic systems, a method has been broadly adopted in which the area gradation is reproduced by a unit obtained by integrating a plurality of pixels using the dither matrix of the AM modulation type. It is to be noted that various methods exist in the form of round dots type, line screen type, chain type as the dither of the AM modulation type. However, the methods are essentially the same in that a plurality of dots are fixed in an arbitrary direction to reproduce the gradation.
Here, to enhance a pseudo gradation number with respect to an arbitrary image to such an extent that the image is visually satisfactory, a basic size of a half-tone dot in the threshold matrix may be set to be large. However, the larger the basic size of the half-tone dot is, the lower the resolution becomes. That is, the resolution disagrees with the gradation in the half-toning process. Therefore, when the half-toning process is performed, and when the gradation is of a satisfactory degree, the image quality is greatly degraded in an edge portion of drawings having the gradation in the image and/or line-like images indicating resolution information.
The resolution which can be output by the image forming apparatus of the electrophotographic system is about 1200 dpi at maximum. This resolution is very low as compared with the printer or the like having a resolution of several thousand dpi. Therefore, to prepare the half-tone dot and reproduce the gradation with respect to an arbitrary angle and line number (about 100 to 200 lines per inch), there is an excessively large geometric restriction, thus the resolution is not suitable for practical use.
It is possible to forcibly prepare a screen while ignoring a geometric position error in digital calculation (development into the raster data). However, when the half-toning is performed using the threshold matrix prepared by the above-described method broadly used in the printing method in the image forming apparatus of the electrophotographic system, many textures are generated with an arbitrary gradation. When many textures are generated due to the influence of many geometric errors of half-tone centers in positions on a two-dimensional plane of an image output onto a final output medium, the textures are visually conspicuous, and a granularity increases (the image quality deteriorates).
As methods for enhancing precision in forming the half-tone dot and visually satisfying the gradation, U.S. Pat. No. 5,155,599 and Jpn. Pat. Appln. KOKAI Publication No. 2003-234900 are known.
In U.S. Pat. No. 5,155,599, a square tile is used for preparing a screen whose angle or line number are closely approximated. A tile parameter (of the square tile) determines the number of half-tone dots included in the tile, and places of the dots. The half-tone dot centers are virtual, in that they are not necessarily positioned on lattices of the images output by the printer. The (virtual) half-tone dot center is used for producing the threshold matrix. That is, when the half-tone dot (virtual) grows from the vicinity of the center of the half-tone dot in the process of calculation of the threshold matrix, the half-tone dot is prepared in the dot position on the output device. In other words, the threshold matrix is set in such a manner that the dot position of the output image formed by the output device is superimposed upon a virtual center position of the half-tone dot. At this time, the dot growth (on the output device) is dithered in order to foresee a level at which coverage (superimposition of the dot position upon the virtual center position) more easily occurs. Therefore, the half-tone dot is not synchronized with the dot growth (on the output device). Instead, the respective half-tone dots are independently grown in a predetermined order.
In Jpn. Pat. Appln. KOKAI Publication No. 2003-234900, and in the ordering described in U.S. Pat. No. 5,155,599, a second virtual half-tone dot center is used, and set to be asymmetric with respect to a periodic duplicate of the first virtual half-tone dot center. In another example, an aggregated distance function is used in the ordering from the virtual half-tone dot center in each virtual half-tone dot center. The aggregated distance function is calculated by using each distance obtained by raising a sum of inverse numbers of distances to the respective virtual half-tone dot centers which have not been already included to a positive power. It has been reported that, in this case, the virtual half-tone dot center having one of minimum values of the sum of the inverse numbers is selected as the next virtual half-tone dot center during the ordering. It has been described as a further example that to define the virtual half-tone dot center of a color different from one selected color, the aggregated distance function used in obtaining the center of the virtual half-tone dot of the selected color is used. Thus, in U.S. Pat. No. 5,155,599 and Jpn. Pat. Appln. KOKAI Publication No. 2003-234900, the problems of the restriction on a geometric shape and the generation of the textures are avoided by a technique referred to as the virtual half-tone dot.
However, in the image forming apparatus of the electrophotographic system, whose outputtable resolution is about 1200 dpi at maximum, phase shifts caused by digital restrictions on a two-dimensional space by the virtual half-tone dots described in U.S. Pat. No. 5,155,599 and Jpn. Pat. Appln. KOKAI Publication No. 2003-234900 cannot be ignored. Therefore, there has been a problem that the subtle periodic shift of the half-tone dot center is visually detected, and roughness or granularity of the image is recognized.